The mammalian brain grows substantially in postnatal life. Despite wide recognition of this phenomenon, little attention has been paid to the cellular basis of ongoing brain growth. To what extent does this growth reflect the addition of new circuitry, and what role might such addition play in some of the major unexplained questions in developmental neurobiology? These enigmas include the basis of critical periods, the age-dependent response of the nervous system to injury, and the remarkable ability of the juvenile brain to store large amounts of new information. In order to successfully address these issues, the formation of novel circuitry in the developing brain must be observed and quantified by evaluating both individual neural elements (axonal branches, dendrites, synapses) and the development of entire circuits. Fortunately, the brains of many mammals comprise complex iterated circuits that can be studied by both morphological and electrophysiological methods. Examples are columns and "blobs" in the visual system, columns and "barrels" in the somatosensory system, glomeruli in the olfactory system, and cell islands and "striasomes" in the neostriatum. Using the developing brain of several different mammalian species --including man-- we will examine systematically the development of two such circuits, olfactory glomeruli and blobs in the primary visual cortex. Part of this work will be carried out using vital imaging, confocal microscopy and magnetic resonance microscopy to examine changes in the arrangement and number of these circuits over time in living animals. In other segments of the project we will use more conventional electrophysiological and morphological approaches to examine the differentiation of neuronal elements and synaptic connections within circuits. As argued in the body of the application, the information that we hope to gain about the postnatal construction of circuitry in the mammalian brain is likely to be of central importance to both basic neurobiology and to understanding human neural development.